Parsing

Isolate a decoder to operate with a fixed number of bytes, and fail if
fewer bytes were consumed, or more bytes were attempted to be consumed.
If the given decoder fails, isolate will also fail.
Offset from bytesRead will be relative to the start of isolate, not the
absolute of the input.

Utility

DEPRECATED. Get the number of bytes of remaining input.
Note that this is an expensive function to use as in order to calculate how
much input remains, all input has to be read and kept in-memory.
The decoder keeps the input as a strict bytestring, so you are likely better
off by calculating the remaining input in another way.